Scientific Abstract Cytochrome P450s (cytP450s) are one of the most important proteins in xenobiotic metabolism. More than ~70% of the drugs in clinical use are metabolized by cytP450s, and the activity of thousands of molecules as substrates for this powerful enzyme is routinely investigated. CytP450s, for instance, are the primary targets for the treatment of prostate and breast cancer. Cytochrome P450 reductase (CPR) plays a crucial role by donating two electrons to cytochrome P450 which is essential for the completion of substrate turnover. However, high-resolution structural information on the protein-protein interaction of the cytP450-CPR complex is still lacking, preventing an in-depth understanding of the fundamental electron-transfer mechanism between the two redox partners. The main objective of the proposed research is to obtain an in-depth understanding of the molecular basis for the effects of CPR on cytP450 activity by determining the structural interactions between the membrane-bound full-length rabbit cytP4502B4 and the membrane-bound FMN binding domain (FBD) of CPR. The structure of the ~90-kDa cytP450-FBD complex embedded in a membrane environment will provide insights into the mechanism by which CPR regulates the enzymatic function of cytP450 and the electron-transfer process between the two proteins. We propose to use a combination of a variety of cutting- edge solution and solid-state NMR experiments to determine the high-resolution structure, dynamics, and membrane orientation of ~30-kDa membrane-bound FMN binding domain of CPR in bicelles first. Then, we will determine the structural and dynamic interactions of the cytP450-FBD complex embedded in bicelles and lipid vesicles. Mutational and functional studies will also be carried out to identify the residues on the interaction interface of the cytP450-FBD complex. The effect of selected substrates on the cytP450-FBD complex will be investigated using NMR and functional measurements. After completing our investigation on rabbit cytP4502B4, if time permitting, we will use other cytochrome P450s (including cytP4503A4, cytP4502D6 and cytP45017A1) to further elucidate the role of the membrane-bound FMN binding domain of CPR on the function of cytochrome P450.